According to standards set by the 3rd Generation Partnership Project (3GPP), user equipment (UE) is required to read a cell's “system information” before selecting it for communication. The 3GPP standard specifies that system information is to be divided into blocks, currently consisting of a Master Information Block (MIB) and one or more Scheduling information Blocks (SB) and System Information Blocks (SIBS).
Fixed-length Broadcast Control CHannel (BCCH) Service Data Unit (SDU) frames are used to convey the various blocks that constitute the system information. Because the various blocks of the system information may be longer than one BCCH frame, the blocks may be segmented and concatenated to fit into the frames. A typical set of system information blocks requires 20 or more BCCH frames to broadcast.
Cells broadcast their system information in a loop, typically repeating the MIB, SB(s) and SIB(s) multiple times in each loop. A typical loop may be 32 or 64 BCCH frames long, and each BCCH frame requires about 20 ms to broadcast. Consequently, a UE typically requires about ½ to one second to acquire a full set of system information.
The system information of a certain cell is subject to change over time. Thus a version number, known as a value tag, is applied to each block of system information. When any of the contents of a block is changed, the value tag of that block is changed, and the value tag of the MIB is changed as well. If the block in which the change occurs happens to be the MIB, only the value tag of the MIB is changed. UEs are notified of a change to system information by paging.